The hydroformylation of allyl alcohol is a well-known and commercially practiced process. See, for example, U.S. Pat. Nos. 4,064,145, 4,215,077, 4,238,419, 4,678,857, and 5,290,743. In the hydroformylation reaction, allyl alcohol is reacted with a CO/H2 gas mixture in the presence of a catalyst to form 4-hydroxybutyraldehyde (HBA). The HBA may then be separated from the catalyst, e.g., by water extraction, and hydrogenated to form 1,4-butanediol (BDO). See U.S. Pat. No. 5,504,261.
Various catalyst systems have been employed for the hydroformylation reaction, most notably a rhodium complex together with a phosphine ligand (see, e.g., U.S. Pat. Nos. 4,064,145, 4,238,419, and 4,567,305). Commonly employed phosphine ligands are trisubstituted phosphines such as triphenyl phosphine. One disadvantage of the hydroformylation process is that other co-products or byproducts are also formed in addition to the desired HBA linear product. The hydroformylation of allyl alcohol typically produces some 3-hydroxy-2-methylpropionaldehyde (HMPA) branched co-product and C3 byproducts such as n-propanol and propionaldehyde. Although HMPA may be hydrogenated to produce 1,3-methyl propanediol (MPD), which is a useful material, the MPD co-product reduces the yield of BDO. Formation of the C3 byproducts effectively represents another yield loss in the process which can have a severe adverse effect on the process economics.
To increase BDO yields, research continues to improve the hydroformylation process and reduce less desired co-product/byproducts. U.S. Pat. No. 6,127,584 discloses that the use of a trialkyl phosphine ligand having at least 2 methyl groups results in increased HBA:HMPA ratio. The use of disphosphine ligands has also been found to improve the HBA:HMPA ratio. The hydroformylation of allyl alcohol using rhodium complex catalysts and disphosphine ligands such as DIOP or trans-1,2-bis(diphenyl-phosphinomethyl)cyclobutane is shown in the art, notably in Japan Kokai Nos. 06-279345 and 06-279344 and U.S. Pat. No. 4,306,087. U.S. Pat. No. 6,225,509 discloses that maintaining the concentration of CO in the reaction liquid above about 4.5 mmols/liter reduces the make of undesirable C3 co-products when using a catalyst comprised of a rhodium complex and a ligand such as DIOP. In addition, co-pending U.S. patent application Ser. No. 11/580,510 discloses that using a 2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis[bis(3,5-di-n-alkylphenyl)phosphino]butane ligand results in a very high yield of 4-hydroxybutyraldehyde compared to 3-hydroxy-2-methylpropionaldehyde.
In sum, new processes for hydroformylating allyl alcohol to produce 4-hydroxybutyraldehyde are needed. Particularly valuable processes would result in high ratios of 4-hydroxybutyraldehyde (HBA) compared to 3-hydroxy-2-methylpropionaldehyde (HMPA).